This invention relates to improvements in a wear plate specifically designed for convenient and removable installation in the pedestal opening of a railway side frame in order to prevent wear on the load bearing surface of the pedestal. Normally, a bearing adapter is provided over the axle bearing, said adapter having a slightly arcuate top surface which bears directly against a corresponding downwardly facing surface in the pedestal opening of the side frame. This arcuate top surface provides the wheel and axle with a freely pivoting end condition to avoid binding loads on the roller bearing.
In service, movement or frictional sliding may occur between the bearing adapter and the pedestal surface, which may cause worn spots on the frame, resulting in loss of the freely pivoting end condition and a weakening of the frame at a load bearing location. Repair of the frame surface is both expensive and time consuming, since the worn surface must be ground down to return it to a flat condition. The amount of grinding allowed is limited by structural considerations; after the limit has been reached the side frame casting is condemned.
To avoid this expensive and time consuming repair, replaceable wear plates may be used between the bearing adapter and the pedestal roof Previous U.S. Pat. Nos. 3,897,736 and 4,203,371, herein incorporated by reference, describe two embodiments of pedestal wear plates. Each of these embodiments constitutes a plate removably attached to the side frame in the pedestal opening comprising a wear surface against the bearing adapter. The plate has resilient side lips which resiliently clamp against the sides of the frame and tend to hold the plate in position during installation and service. Means are provided between the frame and the plate to limit longitudinal movement of the plate relative to the frame, in order to minimize wear on the frame.
As shown in previous U.S. Pat. Nos. 3,897,736 and 4,203,371, installation of the plate requires the plate be forced upwards onto the side frame such that the side lips forceably engage the sidewalls of the frame. So that the plates will be firmly held in place, the lips are separated from one another by a distance that is slightly less than the width of the side frame. This results in a slight elongation and bending of the plate when it is in its installed position, with resultant lateral bending stresses in the plate.
Previous U.S. Pat. No. 4,203,371 introduced a wear plate with a slightly arcuate base to accommodate the plate bending that occurs during installation. When the lips are separated by bending back away from each other and the plate installed, the slightly arcuate plate is flattened for service. Depending on the exact width of the side frame pedestal roof, the lips may need to be separated by slightly different distances from one installation to the next.
The stretching and bending of the plate surface required to install a plate results in lateral bending stresses in the plate surface ("installation bending stresses"). Side frame castings typically have a pedestal roof width tolerance of approximately 0.125". Given a particular side frame that may have a width towards the high end of tolerated widths, an installed wear plate will be stretched to a maximum, with resultant high lateral installation bending stresses. As the plate has a limited capacity to withstand total stress, high lateral installation bending stresses decrease the level of working load stresses that the plate may withstand. The working capacity of the plate therefore varies with the width of the pedestal roof, with particularly wide pedestal roofs resulting in high installation bending stresses and thus increased tendency for plate failure and shortened plate service life.
The plate working load and stress are defined as the frictional forces applied to the wear plate by the bearing adapter as a rail car in service shifts and moves about laterally. These frictional forces induce a tendency for lateral movement of the wear plate, and are opposed by the corresponding friction developed between the plate top and the pedestal roof surface. If this opposing frictional force is insufficient to resist this movement, additional bending load and stress are imposed on the upward side lips of the wear plate. The sensitivity to imbalance in these frictional forces, and hence the tendency to impose stress and load on the plate side lips, increases with heavily loaded rail cars such as coal cars.
The result of the total installation related tensile stresses in combination with stress related to bearing adapter friction can be a significant shortening of the plate service life. In some cases, the total tensile stresses developed may reach the yield strength of the of the plate and thereby cause bending of the plate. In other more severe cases the ultimate strength of the plate may be reached causing cracking.
This problem has been further complicated as of late because of recent frame painting practices. As environmental concerns have caused an effort to reduce volatile organic emissions from sources including paints, rail frames are increasingly being painted with solvent free and alternative solvent based paints. One of the disadvantages of these paints is that a resultant painted surface will have a significantly lower coefficient of friction as compared to a surface painted by older "traditional" paints; at times the new paint may even be thought of as acting as a sort of lubricant. This has the disadvantageous result of greatly reducing the frictional force between the plate top surface and the pedestal roof surface, thereby increasing the effective tensile stress in the plate. This has in turn resulted in an increased occurrence in wear plate bending and cracking.
Increasing the thickness of the plate would seem to offer a means to achieving increased plate strength sufficient to resist lateral movement and consequent failure. The benefits of increasing plate thickness, however, are limited. A practical limit on plate thickness exists as installation bending stresses caused as the side lips are forced apart during plate installation increase in direct proportion to the plate thickness. The difference between these installation bending stresses and the ultimate stress at which failure occurs determine the working capacity of the plate to resist movement. At some thickness a maximum plate working capacity is reached and further thickness increases actually decrease working capacity. Experience has shown a practical plate thickness limit to be approximately 0.109 inches.
For the above stated reasons, an unresolved need exists for a pedestal wear plate with an improved ability to withstand tensile stresses and thereby enjoy a reduced occurrence of bending and cracking.